I love the idea of looking at the ball like a clock face to get better extension with my strokes - this also works well with some of my students. When the ball is over on my right (forehand) side, I think of hitting the ball out at 3 o'clock, and when it's on my left (backhand) side, I try to hit it out at 9 o'clock. This has been especially helpful when looking for a good move with a one-handed backhand.

Keep in mind though that almost no tennis tips are universal and some can actually be sort of a placebo. If one of my kids is swinging wrong and perhaps using too much arm to hit the ball, I might cue them to tune in to the feeling in their legs and feet just to get their heads out of the way of their swings. These sorts of tips are probably the strangest of all.